The aim of these investigative studies is the continuation of research on various aspects of viral carcinogenesis with special reference to simian virus 40 (SV40). This undertaking has been pursued by the Principal Investigator for the past 11 years (1963-1974). In the immediate future the following phenomena will be investigated: (1) The relative role of the target (hamster) cell versus that of the viral (SV40) genome in determining morphogenesis of SV40-induced neoplasms. (2) The interrelationship of the various SV40-mediated cellular "neo-antigens", and their significance in the persistence of the oncogenic state. (3) The importance of the presence of the viral (SV40) genome in the development and severity of oncogenicity in the SV40-hamster cell system.